The ice shelves in Canada's
High Arctic have lost a colossal area this year, scientists report.

The floating
tongues of ice attached to Ellesmere Island, which have lasted for
thousands of years, have seen almost a quarter of their cover break
away.

One of
them, the 50 sq km (20 sq miles) Markham shelf, has completely broken
off to become floating sea-ice.

Researchers
say warm air temperatures and reduced sea-ice conditions in the region
have assisted the break-up.

"These
substantial calving events underscore the rapidity of changes taking
place in the Arctic," said Trent University's Dr Derek Mueller.

"These
changes are irreversible under the present climate."

Scientists reported in July that substantial slabs of ice had calved
from Ward Hunt Ice Shelf, the largest of the Ellesmere shelves.

Similar
changes have been seen in the other four shelves.

As well
as the complete breakaway of the Markham, the Serson shelf lost two
sections totalling an estimated 122 sq km (47 sq miles), and the break-up
of the Ward Hunt has continued.

Cold
remnants

The shelves
themselves are merely remnants of a much larger feature that was once
bounded to Ellesmere Island and covered almost 10,000 sq km (3,500
sq miles).

Over the
past 100 years, this expanse of ice has retreated by 90%, and at the
start of this summer season covered just under 1,000 sq km (400 sq
miles).

Much of
the area was lost during a warm period in the 1930s and 1940s.

Temperatures in the Arctic are now even higher than they were then,
and a period of renewed ice shelf break-up has ensued since 2002.

Unlike
much of the floating sea-ice which comes and goes, the shelves contain
ice that is up to 4,500 years old.

A rapid
sea-ice retreat is being experienced across the Arctic again this
year, affecting both the ice attached to the coast and floating in
the open ocean.

The floating
sea-ice, which would normally keep the shelves hemmed in, has shrunk
to just under five million sq km, the second lowest extent recorded
since the era of satellite measurement began about 30 years ago.

"Reduced
sea-ice conditions and unusually high air temperatures have facilitated
the ice shelf losses this summer," said Dr Luke Copland from
the University of Ottawa.

"And
extensive new cracks across remaining parts of the Ward Hunt Ice Shelf
mean that it will continue to disintegrate in the coming years."

Loss of
ice in the Arctic, and in particular the extensive sea-ice, has global
implications. The "white parasol" at the top of the planet
reflects energy from the Sun straight back out into space, helping
to cool the Earth.

Further
loss of Arctic ice will see radiation absorbed by darker seawater
and snow-free land, potentially warming the Earth's climate at an
even faster rate than current observational data indicates.

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